Keith Duffy, 44, has been taken to hospital while on tour in Bangkok “on the advice of the local medical team”.
Boyzone tweeted: “Keith was taken ill before the gig here in Bangkok… and has been admitted to hospital.”
Fellow Boyzone singer Ronan Keating said Duffy had gone on a “downward spiral” before their performance in Thailand’s capital.
He said Duffy had undergone blood tests but did not yet know the cause of his illness, adding that the band assumed he was unwell because of food poisoning.
Keating told Magic Breakfast that the bandmates had to leave Duffy in hospital to fly to Australia, but his wife Lisa stayed with him.
“I haven’t spoken to him since we left Thailand. We had to leave him in the hospital to get here because we have a show tomorrow night,” he added.
It is unclear whether Duffy will be able to travel to Australia to join them for their next concert on Saturday.
Keating added: “So we’re hoping he can be on a plane tonight in Thailand and be here for the show tomorrow, fingers crossed.”
The group is scheduled to perform in Perth on Saturday, to mark their 25th anniversary.
Keating described the Bangkok gig without Duffy as “hilarious”.
He said: “We had considered certain things, but not others. I had to take some of Keith’s vocals and I didn’t know the words.
“It was in When The Going Gets Tough, and I had no idea, no clue whatsoever.
“I just looked at Shane and handed the mic out to the audience and hoped that they’d sing along.”
The band hopes Duffy will be “right” for their show next Tuesday if he cannot make the Perth concert.
A tweet from the band’s official Twitter account said: “Our brother will be back on his feet as soon as possible and we thank you for all your wonderful support in the meantime.”